These two get used interchangeably in casual conversation, but they test close to opposite things - and preparing for one when you've actually registered for the other is a common way to walk in unprepared.
What a coding contest actually is
A coding contest - Codeforces Rounds, CodeChef Starters, LeetCode Weekly Contest, AtCoder Beginner Contest - gives you a fixed set of well-defined problems with a clear correct answer, and you're scored on how many you solve correctly within a time limit, often with a speed component. There's no ambiguity in what "done" looks like: your submission either passes all test cases or it doesn't. TCS CodeVita follows this exact pattern - open-book rounds with a fixed set of programming problems, scored purely on correctness and completion.
What a hackathon actually is
A hackathon gives you an open-ended or loosely-defined problem and a much longer window (24 to 72+ hours, sometimes longer for build phases) to design, build, and present a working solution. There's no single correct answer - judges evaluate the idea, the execution, the technical soundness of your architecture, and how well you can defend your choices. Adobe's University Hackathon, for instance, moves teams through an assessment, a case-study round, and a full development-and-prototype phase - most of that time isn't spent solving discrete problems with test cases, it's spent making product and architecture decisions with no single right answer.
Where the naming gets genuinely confusing
Some events use "hackathon" in the name for a format that's actually much closer to a coding contest. HackWithInfy is a clear example - despite the name, its main rounds are timed problem-solving on HackerEarth, not a build-a-product hackathon. Always check the actual round structure in a listing rather than assuming from the name alone; a "hackathon" that turns out to be a 3-hour timed coding round needs a completely different kind of preparation than a 36-hour build event.
The skills each one actually rewards
A coding contest rewards fast, correct algorithmic thinking under time pressure - the same muscle a live technical interview screen tests. A hackathon rewards scoping something achievable in the time given, making pragmatic tradeoffs when something breaks with hours left, and communicating a working (if imperfect) result convincingly. Someone strong at one doesn't automatically transfer to the other - plenty of strong hackathon builders would struggle in a timed Codeforces round, and plenty of competitive programmers build unremarkable hackathon projects because the open-endedness itself is the harder part for them.
Deciding which one to prepare for
If you're not sure which an upcoming event actually is, look past the name to three things: is there a single correct answer being scored (contest), or a product being judged on multiple soft criteria (hackathon)? Is the time window measured in hours with no build phase (contest), or does it include research, design, and iteration time (hackathon)? Is scoring automated against test cases (contest), or does a human jury evaluate a presentation (hackathon)?
Where to find either
Brabble's upcoming hackathons feed tracks build-format hackathons specifically. For recurring coding contests, CodeChef Starters, Codeforces Rounds, LeetCode Weekly Contest, and AtCoder Beginner Contest each have a dedicated guide covering format and scoring.